1. Field of the Invention
This invention relates to a wheel-lift device for towing vehicles, and particularly to a device assembled from several parts that surround an axle of a vehicle to be towed, in which the device becomes a rigid appendage to that vehicle and, having that appendage serve as a tongue to hitch to a second vehicle, allows the first vehicle to be towed by the second as a two-wheel cart.
2. Background of the Invention
Wheel-lift towing of vehicles has a history of less than four decades. Wagner, U.S. Pat. No. 3,182,829, May 1965, gave us a vehicular lifting yoke. His yoke taught us to use the axle of the towed vehicle as the horizontal axis for articulation between the towed and towing vehicle. It remains common practice to date.
That design has an inherent problem. The location of the horizontal pivot determines the location of where the weight of the towed vehicle is transferred to the tow truck. That design carries all of the cargo weight some distance behind the axle and the weight of the wheel lift equipment as well is carried behind the truck's axle in most similar designs. Truck builders prefer that a truck's load be carried between the axles or close to the rear axle. A recommended ratio of no more than forty percent of cargo weight should be carried behind the axle.
Earlier wreckers, especially before about 1980, used a tow bar or a sling that attached to the bumper or the end of the frame of the towed vehicle. The weight of the vehicle was carried by the wrecker at the tow bar that had a working position of several feet behind the end of the truck. In comparison that is about half the distance from the axle that many wheel lifts carry their load. To compensate for this shift of the load rearward, wreckers are generally at least one and a half times the length of earlier tow trucks and weigh substantially more.
State commercial drivers license manuals teach drivers that they are responsible for their load and that poor distribution of weight can make vehicle handling unsafe. Tow truck operators are generally required to have commercial drivers licenses and to know these rules. It seems inevitable that towmen will be summoned to court sometime in the future for knowingly operating unsafe vehicles.
The industry seems aware of the challenges it faces carrying towing loads far behind tow trucks' rear axles but it appears to have not yet found a solution that addresses the weight transfer problem. An example appears in the Winter 2002 edition of the Write Carrier & Wrecker Quarterly, Volume 11, Number 1, Page 20, a publication of the Jerr-Dan Corporation, a major manufacturer of towing equipment.
This article describes a user's need for equipment to tow a fire truck with a front axle weight of 21,350 pounds, a front axle set back of 10 feet from the front bumper, and the fire truck having a bucket and snorkel assembly that extends 7 feet ahead of the front bumper. The fire trucks have low hanging components within the underbelly of the fire truck that the towing boom needs to pass under without contact when turning. The manufacturer only approves the axle as a pickup point. The fire trucks are too high to transport on trailers.
This customer's problem invites a comparison between the industry's state of art and the present invention. Their solution was to use a modified heavy duty Underlift carrying the fire truck by the axle using a four stage rigid boom extending behind the truck's tri-axles. The 21,350 pound front axle load of the fire truck is carried about 20 feet behind the center of the tow trucks rear suspension. The rigid lifting boom acts as a lever increasing the 21,350 pound axle weight on the Underlift's axles. Wrecker trucks have a high tare weight. The combined load exceeds the tri-axle legal load weight. The article acknowledges that this combination will be an oversize, over-height, and overweight load.
The present invention shifts both horizontal and vertical axes between the tow truck and the fire truck from the rear of the lifting arm to the front of that arm. The action of the lever on this wheel lift hitch device arm transfers most of the front axle weight to the tow truck, but a percentage of that weight will be transferred to the rear axles of the towed unit. In this example the fire truck could be towed using the present invention by a tandem axle tractor equipped with a pintle hook hitch and be of legal axle weight on the tow truck's axles. The present invention has no arm that pivots under the load. The load may be carried lower because less clearance beneath the truck is required and the towed load may be under legal height limits. A shorter tow truck can be used because it is not necessary to have excessive tow truck length to counter-balance a load carried far behind the axles of the tow truck. The combination can be within legal length requirements.
An additional problem with the present designs of wheel lift tow trucks of all sizes and classes is that all carry the load at the end of a boom behind the truck axle. The boom serves as a lever. A lever amplifies motion and force. That amplified motion can cause movement and forces in excess of design capability of the towed vehicle. An irregular or undulating road surface can cause extreme vertical movement in the suspension of the towed vehicle carried on a wheel lift and retained to that boom and can result in damage to that vehicle.
To avoid some of this damage users can raise the boom to relatively high angles from horizontal to provide more clearance between the boom, its pivot, and drive train components of the towed vehicle. This action causes a different problem. Many tow truck designs advertise a wheel lift boom angle of 10 to 15 degrees above horizontal. When a boom is raised to that angle, the pivot pin between the boom and the transverse bar of the wheel lift also tilts forward at a like angle. The forward tilt of the transverse bar's pivot pin causes the towed vehicle to warp in a turn in relation to the attitude of the tow truck. The actual warp between the inside and outside ends of the transverse bar in a 45 degree turn may be 12 inches or more when the boom is carried at these angles. Cullum, U.S. Pat. No. 5,709,522, January 1998, introduced a double pivot crossbar “capable of rotational movement in both horizontal and vertical planes permitting stress reduction in both the tow truck assembly and the tow truck chassis . . . ” Presumably it may also reduce stress and possible damage to the towed vehicle because cars are never as built as strong as towing equipment.
Available alternatives to a wheel lift tow truck are limited. Car carriers are widely used by professional towing operators. Their size is generally a disadvantage. Three car lengths of space are generally required to load or unload a car carrier compared to two car lengths of space with most wheel lift trucks. That space requirement can limit the efficiency of a car carrier in an urban environment. A second problem with car carriers is the difficulty of damage free loading of a vehicle with front wheels locked at an angle. Force rather than finesse generally prevails and the locked vehicle is dragged onto a truck causing strain on mechanical components. Alm, U.S. Pat. No. 5,779,431, July 1998, introduced a wheel loading device used on a car carrier body to facilitate damage-free loading. It appears to be the only wheel lift design at this time that carries its load primarily between the axles of the truck during transport.
Car dollies also carry vehicles on their own wheels and suspension. Car dollies have only limited capabilities for handling damaged or inoperable vehicles. Most designs include two vertical axes, one at the hitch and a second at the wheels of the towed vehicle. Few operators are able to back up such a combination and commercial use is therefore limited.
A wheel lift device seldom seen in the United States is a truck mounted rotating boom equipped with a rotating lift frame at its outer end. The lift frame is maneuvered over the vehicle and wheel supports attached to the lift frame are placed under each wheel. The vehicle is then hoisted onto the truck body. It appears to be an especially efficient approach for the damage-free removal of illegally parked vehicles.
An under-lift towing device that uses the fifth wheel of a semi-truck tractor as horizontal and vertical axes was introduced to the industry in about 1990. “Tru-Hitch” has arms that lift an axle and extend rearward from that axle to attach to the vehicle frame. Its operation appears to be still new to the industry. The November 2002 issue of Towing & Recovery FOOTNOTES, Volume 14, Number 6, a widely distributed “community newspaper and marketplace for the nation's towers,” pictured a loaded “Tru-Hitch” in a front page article that questioned how it claimed to work without transferring weight from the truck's front axle. The writer, identified as a consultant to the heavy recovery industry, reflects the longstanding industry wide perception that the horizontal pivot can only be located at the axle of the towed vehicle.
There appears to be little precedent in the marketplace or in literature for a wheel-lift for towing that does not use the vehicle wheels as a horizontal axis. A wheel-lift device does not appear to have been used to immobilize wheels to support a fixed tongue. A wheel-lift hitch with a tongue does not appear to have been used to transfer both horizontal and vertical axes to a point ahead of the towed vehicle. Rather the history of wheel-lift towing has been built almost entirely on the aforementioned Wagner patent of 1965.
Devices for lifting a heavy tongue from the ground into a towing hitch on a truck are readily available and in widespread use, especially in the modular and mobile home transporting business. Charles Weber, U.S. Pat. No. 4,000,911, January 1977, introduced a hitch head that was hydraulically adjustable laterally, vertically, and longitudinally. Randall Weber, U.S. Pat. No. 4,946,182, August 1990, followed with an even more versatile boom type hitch. Those, and other hitches with similar capability, are readily adaptable for use with the present invention. Strap winches integral within small booms will offer a convenient and lightweight solution for raising the tongue to a hitch for many users.
The present invention is usually attached to the vehicle to be towed before it is attached to the tow truck so this hitch device conforms to all terrain conditions. The tow truck or the carrier attaches only to the hitch device. Powered wheel lifts often cause damage. Young, U.S. Pat. No. 5,951,235, September 1999, describes problems that all in the industry face and offers a powered solution to it. The present invention, a manually maneuvered hitch device, is powerless to cause damage to a vehicle and can perform similar tasks.
Under the best conditions tow truck operation is seldom without some physical effort. Sophisticated self-loading towing equipment requires the operator to check and perhaps secure the load before departing on a high-speed tow on public roadways. Most towing units still require installation of wheel chocks and retaining straps or chains. The present invention recognizes that fact and includes such tasks in the assembly of the wheel lift hitch onto the vehicle to be towed. Many wreckers and tow trucks now carry towing dollies that require assembly, often at accident scenes. Light duty embodiments of the present invention will be comparable in weight to the advertised weights of towing dollies and will require about the same time and effort to assemble.
It is understood that this device is adaptable to any hitch design that has an adequate vertical load rating in addition to adequate towing capacity. Pintle hooks and a variety of 5th wheel hitches are preferred over most ball hitches that are limited by low vertical load ratings. Gooseneck hitch adaptors can be used with models that have an adjustable length tongue and will offer better load transfer at some cost to convenience.